1. Field of the Invention
An extensive literature has been developed demonstrating the ability to produce polypeptide sequences in a wide variety of cellular hosts. Numerous genes have been isolated from mammals and viruses, joined to transcriptional and translational initiation and termination regulatory signals from a source other than the structural gene and introduced into hosts in which the regulatory signals are functional. Frequently, the peptide of interest is prepared to be used for a physiological purpose. In many situations, physiological activity requires not only having the correct or substantially correct amino acid sequence, but the peptide must fold properly including proper disulfide linkage formation and may require further processing, such as acetylation, glycosylation or methylation.
For economic production, one would wish to use unicellular microorganisms, which could be grown in large fermentation tanks, do not have fastidious nutrient requirements and are relatively economical to maintain. Bacteria, such as E. coli, B. subtilis, or the like, fungi, such as yeast, Candida, filamentous fungi, or the like, offer economic opportunities to produce a wide variety of peptides. However, because of the substantial difference in the nature of the unicellular microorganisms and mammalian cells, the folding and processing in a mammalian cell appears to be substantially different from these lower order organisms. Therefore, the products which are obtained from the unicellular microorganisms may not have been properly processed or folded so as to realize a substantial proportion or all of the physiological activity of the naturally occurring peptide obtained from a native host.
There therefore remains substantial interest in providing alternative economic systems for producing peptides, where high yields may be obtained and significantly, the products may be produced in a form providing for a high degree of physiological activity common to the wild-type peptide having the same or substantially the same amino acid sequence.